Polyvinylacetal resin and process for producing the same

ABSTRACT

A polyvinylacetal resin powder with a low metal content, being excellent in transparency, moisture resistance and electrical insulating properties and having a large specific surface area, and a process for its production, are presented. A polyvinylacetal resin powder obtained by reacting polyvinyl alcohol and an aldehyde in the presence of an acid catalyst, which has an acetalization degree of at least 60 mol %, a specific surface area of from 1.50 to 3.50 m 2 /g, a bulk density of from 0.12 to 0.19 g/cm 3 , an average particle diameter of from 0.5 to 2, 5 μm and a metal content of at most 80 ppm. 
     It is produced, for example, by feeding a reaction fluid containing polyvinyl alcohol, an aldehyde and an acid catalyst into a first reactor, to carry out an acetalization reaction, discharging the reaction fluid wherein the acetalization degree has reached from 10 to 60 mol %, and feeding the same into a second reactor to carry out a further reaction to bring the acetalization degree of polyvinylacetal to at least 65 mol %.

TECHNICAL FIELD

The present invention relates to a porous polyvinylacetal resin whichhas a low content of a metal component such as an alkali metal and whichis excellent in transparency, moisture resistance, electrical insulatingproperties, etc. and has a large specific surface area, and a novelprocess for its production.

BACKGROUND ART

Polyvinylacetal resins are widely used for various coating materials,adhesives, binders and molded products. Heretofore, for the productionof a polyvinylacetal resin, it has been common to employ a process whichcomprises reacting polyvinyl alcohol and an aldehyde in an aqueoussolution in the presence of an acid catalyst, neutralizing the resultingresin slurry of polyvinylacetal resin with an alkali, followed bydehydration, washing and then drying to obtain it in the form of apowder. Further, it is common to employ a process which comprisesreacting polyvinyl alcohol and an aldehyde in an aqueous solution in thepresence of an acid catalyst till the desired final acetalization degreein one step, neutralizing the obtained resin slurry with an alkali,followed by dehydration, washing and then drying to obtain it in theform of a powder.

In such a case, an alkali neutralizing agent such as sodium hydroxideused for neutralization, will react with the acid catalyst to form ametal salt. Such a metal salt, an unreacted alkali neutralizing agentand an unreacted acid catalyst (hereinafter referred to as metalcomponents) will be taken into particles of the polyvinylacetal resin orwill deposit on the surface of particles of the resin. Such metal(alkali) components may be removed to some extent by repeating washingwith water, but usually, it is difficult to remove metal componentstaken into particles of the resin.

A metal component remaining in the resin, such as an alkali metal islikely to impair the characteristics of the polyvinylacetal resin, suchas the transparency, moisture resistance, electrical insulatingproperties, etc. and thus brings about a problem from the viewpoint ofthe product quality especially in an application to a molded product forwhich a high level of transparency or moisture resistance is required orto an adhesive for electronic materials for which electrical insulatingproperties are required. Thus, an improvement is required in thisrespect. In order to solve such a quality problem of the transparency,moisture resistance, electrical insulating properties, etc. caused by analkali, various proposals have been made from the viewpoint of thereaction scheme or the production method. For example, there are amethod of adding an alkylene oxide when the predetermined acetalizationdegree has been reached, to let it reacted with the remaining acidcatalyst to terminate the acetalization reaction (e.g. JP-A-4-55404), amethod of carrying out the acetalization reaction and precipitation withvigorous stirring (e.g. JP-A-11-349629), a method of employing aloop-shaped reactor (e.g. JP-A-5-59117), a method of employing a reactorhaving a flat smooth surface (e.g. JP-A-4-275310), a method of employinga reactor made of a corrosion-resistant material (e.g. JP-A-5-140216), amethod of neutralizing the reaction product slurry while applyingultrasonic vibration (e.g. JP-A-5-97919) and a method of precipitatingthe reaction product slurry in a powder form, followed by purificationby electrodialysis (e.g. JP-A-2000-38456).

Further, the polyvinylacetal resin is likely to adhere various materialssuch as metals, plastics or glass, and in its production in anindustrial scale, it tends to stick to the interior of the reactor orpipings, which tends to cause a serious technical problem.

With a polyvinylbutyral resin synthesized by a conventional process, ithas not been sufficient to remove a metal component in the resin bywashing, and a polyvinylacetal resin which is excellent in removabilityof a metal component by washing and which has a low content of a metalcomponent, and a process for its production, have been desired. Further,a production process free from sticking in the interior of the reactoror pipings, has been desired. The present inventors have conducted aresearch and development to meet with such demands, and as a result,have found a polyvinylacetal resin which is excellent in theremovability by washing of an alkali metal component such as sodiumremaining in the resin and which scarcely sticks to the productionequipment such as a reactor or pipings and has a low content of a metalcomponent, and a process for its production. The present invention hasbeen accomplished on the basis of these discoveries.

DISCLOSURE OF THE INVENTION

The present invention is characterized by having the following gists.

-   1. A polyvinylacetal resin characterized in that it is obtained by    reacting polyvinyl alcohol and an aldehyde in the presence of an    acid catalyst, and it has an acetalization degree of at least 60 mol    % and a specific surface area of from 1.50 to 3.50 m²/g.-   2. The resin according to 1, which has a bulk density of from 0.12    to 0.19 g/cm³.-   3. The resin according to 1 or 2, which has an average particle    diameter of from 0.5 to 2.5 μm.-   4. The resin according to 1, 2 or 3, which has a metal content of at    most 80 ppm.-   5. A process for producing a polyvinylacetal resin, characterized by    feeding a reaction fluid containing polyvinyl alcohol, an aldehyde    and an acid catalyst into a first reactor, to carry out an    acetalization reaction, discharging the reaction fluid wherein the    acetalization degree has reached from 10 to 60 mol %, and feeding    the same into a second reactor to carry out a further reaction to    bring the acetalization degree of polyvinylacetal to at least 65 mol    %.-   6. The process for producing a polyvinylacetal resin according to 5,    wherein the first reactor is a closed reactor provided with a    stirring mechanism.-   7. The process for producing a polyvinylacetal resin according to 5    or 6, wherein the reaction temperature in the first reactor is    within a range of from 10 to 60° C.-   8. A process for producing a polyvinylacetal resin, characterized by    comprising steps of continuously feeding polyvinyl alcohol, an    aldehyde and an acid catalyst into a closed reactor, to carry out an    acetalization reaction, and continuously discharging a reaction    product wherein the acetalization degree of polyvinylacetal formed,    has reached at least 10 mol %, out from the closed reactor.-   9. The process according to 8, which comprises steps of further    aging and reacting the reaction product continuously discharged out    from the closed reactor, in a separate reactor, and after the    acetalization degree of polyvinylacetal has reached at least 60%,    subjecting this polyvinylacetal to neutralization, washing with    water, dehydration and drying.-   10. The process according to 8 or 9, wherein the reaction    temperature in the closed reactor is within a range of from 20 to    50° C.-   11. A process for producing a polyvinylacetal resin, characterized    by continuously or intermittently feeding a reaction fluid    containing polyvinyl alcohol, an aldehyde and an acid catalyst into    a reactor, to carry out an acetalization reaction, so that the    average retention time of the reaction fluid in the reactor would be    at least 30 minutes, and continuously or intermittently discharging    the reaction fluid wherein the acetalization degree has reached at    least 10 mol % and less than 65 mol %, out from the reactor.-   12. The process according to 11, wherein the reaction fluid    continuously or intermittently discharged out from the reactor, is    further reacted in a separate reactor, to bring the acetalization    degree of polyvinylacetal to at least 65%.-   13. The process according to 11 or 12, wherein the reactor is a    closed reactor provided with a stirring mechanism.-   14. The process according to 11, 12 or 13, wherein the reaction    temperature in the reactor is within a range of from 20 to 50° C.-   15. A process for producing a polyvinylacetal resin, which comprises    reacting polyvinyl alcohol and an aldehyde in the presence of an    acid catalyst, characterized by feeding polyvinyl alcohol, an    aldehyde and an acid catalyst into a reactor of a first reaction    apparatus to carry out an acetalization reaction, discharging a    first reaction fluid wherein the acetalization degree has reached at    least 10 mol % from the reactor of the first reaction apparatus, and    then feeding this first reaction fluid into a second reaction    apparatus having one reactor or two or more reactors connected in    series to carry out aging and a reaction to bring the acetalization    degree to at least 60 mol %.-   16. The process according to 15, wherein at least one reactor among    the reactors of the first reaction apparatus and the second reaction    apparatus is a closed reactor provided with a stirring mechanism.-   17. The process according to 15 or 16, wherein the average retention    time of the reaction fluid in the reactor of the first reaction    apparatus is at least 1 minute.-   18. The process according to any one of 15 to 17, wherein the total    of the average retention times of the reaction fluid in the reactors    of the second reaction apparatus satisfies the following formula:    εVi/Q≧1    where the total number of reactors of the second reaction apparatus    is N (number), Vi is the volume (liters) of the i-th reactor in the    second reaction apparatus, i is an integer of from 1 to N, εVi is    the total of the volumes of the respective reactors of the second    reaction apparatus, and Q is the amount per unit time (liters/hr) of    the first reaction fluid flowing into the second reaction apparatus.-   19. A process for producing a polyvinylacetal resin, which comprises    reacting polyvinyl alcohol and an aldehyde in the presence of an    acid catalyst, characterized by comprising steps of preliminarily    charging water or an aqueous solution having an acid catalyst    dissolved therein into a reactor, then feeding polyvinyl alcohol, an    aldehyde and an acid catalyst to the reactor, then stopping the    feeding of the raw materials, and then carrying out an acetalization    reaction and an aging reaction in the reactor to bring the    acetalization degree of polyvinylacetal to at least 60 mol %.-   20. The process according to 19, wherein the volume of the water or    the aqueous solution having an acid catalyst dissolved therein,    preliminarily charged into the reactor, and the volume per unit time    of the polyvinyl alcohol, the aldehyde and the acid catalyst, fed    into the reactor, satisfy the following formula:    V/v≧0.5    where V is the volume (liters) of the water or the aqueous solution    having an acid catalyst dissolved therein, preliminarily charged    into the reactor, and v is the volume per unit time (liters/hr) of    the polyvinyl alcohol, the aldehyde and the acid catalyst, fed into    the reactor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view schematically illustrating the relation between thefirst reaction apparatus and the second reaction apparatus in oneembodiment of the present invention. Here, in Fig, the method of feedingraw materials into the reactor of the first reaction apparatus and themethod of feeding the reaction fluid into the reactor of the secondreaction apparatus, are shown to be a system wherein the material is fedfrom a lower portion of the reactor and discharged from an upper portionof the reactor.

EXPLANATION OF SYMBOLS

10: First reaction apparatus

11: Reactor of the first reaction apparatus

12: Aqueous polyvinyl alcohol solution

13: Butyl aldehyde

14: Hydrochloric acid

15: First reaction fluid

16: Second reaction fluid

20: Second reaction apparatus

21: First reactor of the second reaction apparatus

22: Second reactor of the second reaction apparatus

23: Nth reactor of the second reaction apparatus

MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described in detail. In the followingdescription, “parts” and “%” are represented as based on mass unlessotherwise specified.

The polyvinylacetal resin in the present invention is characterized inthat it is preferably a porous powder which is obtainable by reactingpolyvinyl alcohol and an aldehyde in the presence of an acid catalystand which has an acetalization degree of at least 60 mol %, preferablyat least 65 mol % and a large specific surface area. The polyvinylacetalresin of the present invention has a specific surface area of the resinas measured by a mercury injection method within a range of from 1.5 to3.5 m²/g, preferably from 1.7 to 3.0 m²/g. If it is less than 1.5 m²/g,such will be close to the characteristics of a common polyvinylacetalresin, and if it exceeds 3.5 m²/g, the resin tends to float, wherebyloss tends to increase in the washing or dehydration step, such beingundesirable. The specific surface area was measured by the followingprocedure under the following conditions by means of an automaticporosimeter autopore IV500, manufactured by Shimadzu Corporation.Firstly, about 0.6 g of a sample was taken into a sample cell, weighedand then set in an apparatus. Then, it was subjected to vacuumevacuation treatment to 50 μg (6.7 Pa) in the apparatus and thenmeasured. The measurement was carried out under such conditions that themercury injection pressure was 1 psia (6900 Pa), the maximum mercuryhead pressure was 44500 psia (290 MPa), and the equilibrium time was 10seconds.

Further, the bulk density of the polyvinylacetal resin of the presentinvention is preferably from 0.12 to 0.19 g/cm³, particularly preferablyfrom 0.14 to 0.17 g/cm³. The bulk density was measured in the followingprocedure. Into a measuring cylinder having a capacity of 200 mL and apreliminarily known weight, the powder of the polyvinylacetal resin wasput, and after adjusting the top surface accurately, the weight wasmeasured. The difference in weight (the weight of the powder present inthe measuring cylinder) was divided by the volume (200 mL) of themeasuring cylinder to obtain the bulk density.

The average particle diameter of the polyvinylacetal resin of thepresent invention is preferably from 0.5 to 2.5 μm, particularlypreferably from 0.8 to 2 μm. The primary particle diameters weremeasured with respect to at least particles of n=at least 20 on thebasis of a scanning electron microscopic photograph taken under 5000magnifications, and an average particle diameter is calculated fromthese measured results.

The polyvinylacetal resin of the present invention has a large specificsurface area and is excellent in removability of a metal component bywashing, and accordingly, the content of a metal component (particularlyan alkali metal such as sodium) is very low at a level of at most 80ppm, particularly preferably at most 60 ppm. Accordingly, it isexcellent in moisture resistance, transparency, electrical insulatingproperties, etc., and thus can be suitably used as a raw material resinfor various coating materials, adhesives, binders or molded products. Tothe polyvinylacetal resin of the present invention, a plasticizer, alubricant, a filler, a stabilizer, etc. may suitably selectively beadded. As a method of incorporating additives, an optional method may beemployed which is commonly used in the resin processing field. Forexample, a closed mixer such as mixing roll or a kneader, or an extruderhaving a kneading function may be used.

The powder of the polyvinylacetal resin of the present invention ispreferably produced as follows.

As the polyvinyl alcohol as the raw material to be used in the presentinvention, one having an average degree of polymerization of from 200 to4000 and saponification degree of at least 80%, is used. Further, in thepresent invention, the polyvinyl alcohol is used in the form of a 3 to15% aqueous solution, preferably a 5 to 12% aqueous solution, in orderto feed it continuously and constantly into the reactor.

The aldehyde as the second raw material to be used in the presentinvention, may be an aldehyde which is commonly used as a raw materialfor the synthesis of a polyvinylacetal resin. For example, it may be analiphatic aldehyde such as formaldehyde, acetaldehyde, propionaldehyde,n-butyl aldehyde, tert-butyl aldehyde, amyl aldehyde, hexyl aldehyde or2-ethylhexyl aldehyde, an alicyclic aldehyde such as cyclohexyl aldehydeor furfural, or an aromatic aldehyde, such as benzaldehyde, analkyl-substituted benzaldehyde or a halogen-substituted benzaldehyde, ora phenyl-substituted alkylaldehyde. Among them, acetaldehyde or butylaldehyde is preferably employed. These aldehydes may be used alone or incombination as a mixture of two or more of them.

As the acid catalyst to be used in the present invention, hydrochloricacid, phosphoric acid, sulfuric acid, citric acid and p-toluene sulfonicacid may, for example, be used alone or in combination as a mixture oftwo or more of them. Such an acid catalyst is added usually in asuitable amount so that the pH of the reaction fluid will be from 0.3 to2.0.

Specifically, the polyvinylacetal resin in the present invention may beproduced by the following preferred embodiments (a) to (d).

(a) In this embodiment, polyvinyl alcohol, an aldehyde and an acidcatalyst are continuously fed into a closed reactor to carry out anacetalization reaction, and after the acetalization degree has reachedat least 10%, the reaction product is continuously discharged, and thenaged and reacted in a separate reactor, followed by neutralization,washing with water, dehydration and drying.

Namely, this embodiment is characterized in that three raw materialsi.e. polyvinyl alcohol, an aldehyde and an acid catalyst, are fed into aclosed reactor provided with an inlet for the raw materials, and afterthe acetalization degree after the initiation of the reaction, hasreached at least 10 mol %, the reaction fluid is discharged continuouslyout from the reactor.

By continuously discharging the reaction fluid out of the reactionsystem in such a manner, the product formed by the reaction (hereinafterreferred to as the reaction product) will be one having high porosity.Further, by transferring the reaction fluid to a reactor for aging tocomplete the reaction, it is possible to obtain a polyvinylacetal resinwhich has a porosity heretofore not available and which is excellent inremovability of a metal component by washing.

The method of feeding the raw materials is not particularly limited, andit is possible to employ {circle around (1)} a method of feeding thethree types from separate inlets respectively, {circle around (2)} amethod of preliminarily mixing the polyvinyl alcohol and the acidcatalyst and then feeding the aldehyde and the mixed fluid of thepolyvinyl alcohol and the acid catalyst, separately from inlets, or{circle around (3)} a method of preliminarily mixing the polyvinylalcohol and the aldehyde, and then feeding the acid catalyst and themixed fluid of the polyvinyl alcohol and the aldehyde, separately frominlets. Among them, {circle around (1)} or {circle around (2)} ispreferred from the viewpoint of the control of the reaction.

Discharging of the reaction product from the reactor is carried outcontinuously. In such a case, it is preferred to use a reactor andpiping having smooth surface finish applied to a welded portion betweenthe piping and the reactor or to the inner wall of the piping so thatthe reaction product will not locally or macroscopically stay in thevicinity of the discharge outlet of the reactor or in the dischargepiping. More specifically, preferred is, for example, a method offeeding the raw materials from an upper portion of the reactor andcontinuously discharging the reaction product from a lower portion ofthe reactor, a method of feeding the raw materials to a lower portion ofthe reactor by means of a supply tube from an upper portion of thereactor and continuously discharging the reaction product from the upperportion of the reactor, or a method of feeding the raw materials from alower portion of the reactor and continuously discharging the reactionproduct from an upper portion of the reactor. Further, a method whereinwater is preliminarily filled in the reactor and then the raw materialsare fed, is effective as a method to prevent pooling of air in thereactor.

The reactor is preferably a known tank type reactor provided with astirring mechanism, or a tubular reactor, with a view to reacting thethree types of raw materials in a homogeneous system. As the stirringcondition, with a view to accomplishing sufficient stirring, it ispreferred to carry out the stirring with a stirring power per unitvolume of at least 0.4 kW/m³.

As stirring vanes to be used for stirring the reaction fluid in thereactor, conventional ones may be used. For example, three-bladed sweptback vanes, paddle vanes, anchor vanes, max blend vanes and full zonevanes may be mentioned. With a view to accomplishing sufficient mixing,it is preferred to use so-called large size vanes such as max blendvanes or full zone vanes.

Usually, the reaction temperature for the acetalization reaction is setwithin a range of from 0 to 90° C. However, usually, it is not common tocarry out the acetalization reaction in the vicinity of the center ofthis range (20 to 50° C.). It is rather common to carry out the reactionat a low temperature of at most 10° C. or, inversely and intentionally,at a high temperature of at least 60° C., in order to obtain a resinexcellent in washing properties. On the other hand, in the presentinvention, for the acetalization reaction, a temperature of from 10 to60° C., preferably from 20 to 50° C., particularly preferably from 25 to45° C., is selected. It is also one of characteristics of the presentinvention that by such a reaction at an intermediate temperature of from20 to 50° C., it is possible to obtain a porous resin (having a largespecific surface area) having good washing properties, which isexcellent in washability of a metal component, and whereby the contentof the metal component in the resin can relatively easily be lowered tothe desired concentration.

In the present invention, the reaction time, i.e. the time during whichthe raw materials fed into the reactor remain in the reactor until theyare discharged (the average retention time), is also characteristic.This reaction time is selected for every reaction temperature conditionso that the acetalization degree will be at least 10 mol %, preferablyat least 15 mol %, further preferably at least 20 mol %. Depending uponthe reaction temperature, the time until the desired acetalizationdegree will be reached, varies, but as an index, it may be selectedwithin a range of from 10 seconds to 7 minutes.

The reaction product discharged from the reactor is then transferred toanother reactor to carry out an aging reaction. The reaction temperaturefor the aging is within a range of from room temperature to 90° C.,preferably from 30 to 70° C., and the reaction time is set so that theacetalization reaction reaches and completes to a desired acetalizationdegree and is usually set within a range of from 1 to 24 hours,preferably from 1 to 10 hours. The acetalization degree desired by thisaging reaction is selected to be at least 60 mol %, preferably at least65 mol %.

The aging reaction product (slurry) thus obtained exhibits acidity dueto the acid catalyst. To neutralize this slurry, an alkali neutralizingagent such as sodium hydroxide or sodium bicarbonate will be added.Usually, the pH is adjusted to be from 7 to 11.

Then, dehydration and washing with water are repeated to remove themetal component remaining on the surface or void spaces of the powder.In addition, the remaining acid catalyst and the reaction residues suchas the aldehyde, will also be removed. Such washing with water iscarried out at a temperature of from room temperature to 60° C. Usually,the temperature is preferably at least 40° C., but the resin powderobtained by the present invention has a large specific surface area andthus has sufficient washability even by washing with water at roomtemperature.

The drying method is not particularly limited. For example, a knownmethod such as a vacuum drying method or a hot air circulating dryingmethod, may be employed.

(b) In this embodiment, polyvinyl alcohol, an aldehyde and an acidcatalyst are continuously or intermittently fed into a reactor to carryout an acetalization reaction, and a reaction fluid wherein theacetalization degree has reached at least 10 mol % and less than 65 mol%, is continuously or intermittently discharged and then subjected to anaging reaction in another reactor, followed by neutralization, washingwith water, dehydration and drying to obtain the resin.

Namely, the volume of the reactor and the volume of the raw materials tobe fed per unit time, are selected so that the average retention time ofthe reaction fluid in the reactor will be preferably at least 30minutes, and the reaction fluid containing the polyvinyl alcohol, thealdehyde and the acid catalyst in the reactor, is continuously orintermittently discharged out from the reactor. Here, the averageretention time is represented by V/v where V is the volume (liters) ofthe reactor, and v is the volume per unit time (liters/min) of the rawmaterials fed. Namely, by continuously or intermittently discharging thereaction fluid from the reaction system under a certain condition inthis manner, the reaction product (hereinafter referred to as thereaction fluid) will have a remarkable porosity, i.e. a specific surfacearea of particles being at least 1.5 m²/g, and sticking to the interiorof the reactor will be substantially reduced.

The method of feeding the raw materials is not particularly limited, andthe same method as disclosed in the above (a) can be adopted, and method{circle around (1)} or {circle around (2)} is preferred from theviewpoint of the control of the reaction.

Discharging of the reaction product from the reactor is carried outcontinuously, and the same reactor, piping, system, etc. as disclosed inthe above (a) may be used. Further, in a case where an open type reactorhaving a gas-liquid interface at the top of the reaction fluid, isemployed, it is possible to use, for example, a method of feeding theraw materials from a lower portion of the reactor and discharging thereaction fluid from the side of the reactor, or a method of feeding theraw materials from an upper portion of the reactor and discharging thereaction fluid from a lower portion of the reactor so that the reactionfluid level will be constant.

With a view to reacting the three types of raw materials in ahomogeneous system, the reactor is preferably a tank type reactorprovided with a stirring mechanism, or a tubular reactor, and as astirring condition, with a view to accomplishing proper mixing, it ispreferred to carry out the stirring with a stirring power per unitvolume of at least 0.05 kW/m³. Particularly in a case where an open-typereactor is employed, it is preferred to carry out stirring at arelatively low rotational speed with little change of the liquidsurface, with a view to suppressing sticking at the gas-liquid interfaceof the reaction fluid.

As stirring vanes to be used for stirring the interior of the reactor,those exemplified in the above (a) may be employed. Particularly with aview to reducing sticking of the resin to the stirring vanes or thestirring shaft, it is preferred to employ a lower portion stirringsystem. Further, with a view to reducing sticking of the resin to abaffle, a lower baffle system is preferred. The reaction temperature forthe acetalization reaction is preferably from 20 to 50° C., as disclosedin the above (a), whereby a resin having good washability can beobtained.

Next, in the present invention, the average retention time, i.e. theaverage time during which the raw materials fed into the reactor remainin the reactor until they are discharged, is important. The volume ofthe reactor and the volume per unit time of the raw materials to be fed,are set so that the average retention time will be at least 30 minutes,preferably at least 45 minutes, more preferably at least 60 minutes.Further, the polyvinylacetal resin of the present invention can beobtained when the average retention time is set to be at least 30minutes. It is undesirable from the viewpoint of the productionefficiency to prolong the average retention time unnecessarily (e.g. atleast two hours). The acetalization degree of the reaction fluiddischarged is preferably at least 10 mol %, preferably at least 40 mol%, more preferably at least 55 mol % and less than 65%, although it mayvary depending upon the reaction temperature.

The reaction fluid (slurry) discharged from the reactor is thentransferred to another reactor to carry out an aging reaction. The agingreaction temperature, time and desired acetalization degree are the sameas disclosed in the above (a). The treatment for neutralization of theslurry thus obtained, the treatment for removing a metal component bydehydration and washing with water, and the drying method, are also thesame as disclosed in the above (a).

It is also one of the characteristics of this production process thatsticking of the resin in the interior of the reactor, the stirring vanesand the interior of the piping, is scarce. The sticking state in thereactor was judged by visual observation by disassembling the reactorafter completion of the reaction. Further, the deposits on the innersurface of the reactor, on the stirring vanes and stirring shaft and onthe baffle, were peeled off in their entire amount, and the total weightwas measured.

(c) In this embodiment, an aqueous polyvinyl alcohol solution, analdehyde and an acid catalyst are preferably continuously fed to areactor of a first reaction apparatus to carry out an acetalizationreaction so that the acetalization degree of a first reaction fluid willbe at least 10 mol %, and the first reaction fluid is preferablycontinuously discharged from the reactor of the first reactionapparatus. Then, the first reaction fluid is preferably continuously fedinto a reactor of a second reaction apparatus having one reactor or twoor more reactors connected in series, to carry out an aging reaction toform a second reaction fluid in which the acetalization degree is atmost 60 mol %, and then the second reaction fluid is preferablycontinuously discharged from the second reaction apparatus.

Namely, here, firstly, in the first reaction apparatus, the aqueouspolyvinyl alcohol solution, the aldehyde and the acid catalyst arepreferably continuously fed into the reactor provided with an inlet tofeed the raw materials to carry out the acetalization reaction so thatthe acetalization degree will be at least 10 mol %, and the firstreaction fluid is preferably continuously discharged from the reactor ofthe first reaction apparatus.

The method of feeding the aqueous polyvinyl alcohol solution, thealdehyde and the acid catalyst into the reactor of the first reactionapparatus, is not particularly limited, and it is possible to employ,for example, {circle around (1)} a method of feeding the aqueouspolyvinyl alcohol solution, the aldehyde and the acid catalyst fromthree inlets separately, {circle around (2)} a method of preliminarilymixing the polyvinyl alcohol and the acid catalyst and then feeding thealdehyde and the mixed fluid of the polyvinyl alcohol and the acidcatalysts, from separate inlets, or {circle around (3)} a method ofpreliminarily mixing the polyvinyl alcohol and the aldehyde, and thenfeeding the acid catalyst and the mixed fluid of the polyvinyl alcoholand the aldehyde, from separate inlets. Among them, from the viewpointof the control of the reaction, the methods {circle around (1)} and{circle around (2)} are preferred.

Discharging of the first reaction fluid from the reactor of the firstreaction apparatus is carried out preferably continuously. In such acase, it is preferred to use the reactor and piping having smoothsurface finish applied to a welded portion of the piping and the reactoror to the inner wall of the piping, so that the reaction fluid will notstay locally or macroscopically in the vicinity of the discharge outletof the reactor or in the discharge piping. As the method for feeding ordischarging, it is preferred to employ {circle around (1)} a method offeeding the raw materials from an upper portion of the reactor of thefirst reaction apparatus and discharging the reaction fluid from a lowerportion of the reactor, {circle around (2)} a method of feeding the rawmaterials into a lower layer portion of the reactor of the firstreaction apparatus by means of a supply tube from an upper portion ofthe reactor of the first reaction apparatus and discharging the reactionfluid from an upper portion of the reactor of the first reactionapparatus, or {circle around (3)} a method of supplying the rawmaterials from a lower portion of the reactor of the first reactionapparatus and discharging the reaction fluid from an upper portion ofthe reactor of the first reaction apparatus. Further, a method ofpreliminarily filling water into the reactor of the first reactionapparatus and then feeding the raw materials, is effective as a methodto prevent pooling of air in the reactor.

The reactor of the first reaction apparatus is preferably a tank typeclosed reactor provided with a stirring mechanism for the purpose ofuniformly mixing the polyvinyl alcohol, the aldehyde and the acidcatalyst to be fed and suppressing sticking of the obtained firstreaction fluid on the inner wall of the reactor. As a stirringcondition, with a view to accomplishing sufficient mixing, it ispreferred to adjust the stirring power per unit volume to be at least0.4 kW/m³.

As stirring vanes to be used for stirring the first reaction fluid inthe reactor of the first reaction apparatus, those exemplified in theabove (a) may be used. Further, the reaction temperature for theacetalization reaction is preferably from 20 to 50° C., as mentioned inthe above (a), whereby a resin having good washability can be obtained.

The reaction time of the first reaction fluid in the reactor of thefirst reaction apparatus, i.e. the time until the raw materials fed intothe reactor will be discharged as the first reaction fluid (the averageretention time), is set to be at least one minute, preferably at least1.5 minutes, although it depends on the reaction temperature. Bysecuring such an average retention time, the acetalization reaction willproceed in the reactor of the first reaction apparatus, and the reactionproduct can be discharged as the first reaction fluid.

Then, the first reaction fluid continuously discharged from the firstreaction apparatus is preferably continuously fed into the secondreaction apparatus having one reactor or two or more reactors connectedin series, to carry out the aging reaction, so that the acetalizationdegree of the second reaction fluid in the second reaction apparatuswill be at least 60 mol %, preferably at least 65 mol %, and the secondreaction fluid is preferably continuously discharged from the secondreaction apparatus.

In order to facilitate the aging reaction, it is preferred to set thereaction temperature high, but sticking on the inner wall of the reactoris likely to be thereby caused. Accordingly, from the viewpoint of theaging reaction rate and the suppression of such sticking, thetemperature in the reactor of the second reaction apparatus is usuallyfrom room temperature to 70° C., preferably from 30 to 60° C., morepreferably from 35 to 55° C.

In the present invention, the aging reaction in the reactor of thesecond reaction apparatus is preferably continuously carried out. It isso designed that the aging reaction time, i.e. the total of the averageretention times in the respective reactors of the second reactionapparatus, will satisfy the following relation.εVi/Q≧1wherein in the above formula, Q is the amount per unit time (liters/hr)of the first reaction fluid, the total number of reactors in the secondreaction apparatus is N (number), Vi is the volume (liters) of the i-threactor in the second reaction apparatus, i is an integer of from 1 toN, and εVi is the total of the respective volumes of the reactors in thesecond reaction apparatus.

Namely, the volumes and the number of the reactors in the secondreaction apparatus are determined so that the total of the respectiveaverage retention times in the reactors in the second reaction apparatus(i.e. the aging reaction time) will be at least one hour. For example,in a case where the amount fed into or discharged from the reactor inthe second reaction apparatus is 30 liters/hr, it is possible to employone reactor in the second reaction apparatus, having an internalcapacity of 30 liters or two reactors in the second reaction apparatus,each having an internal capacity of 15 liters, or three reactors in thesecond reaction apparatus, each having an internal capacity of 10liters, or five reactors in the second reaction apparatus, each havingan internal capacity of 6 liters. In a case where a large size of areactor in the second reaction apparatus to be used for the agingreaction is problematic, if the process of the present invention isused, it is possible to secure a predetermined aging reaction time byusing a plurality of reactors having small internal capacities, wherebymerits are substantial, such as compact installation and increase in thefreeness in design.

The reactors of the second reaction apparatus to carry out the agingreaction are preferably closed tank type reactors provided with astirring mechanism with a view to reacting the first reaction fluid fedinto the reactors of the second reaction apparatus in a uniform stateand with a view to preventing sticking to the inner walls of thereactors. Further, the method of feeding the reaction fluid to a reactorin the second reaction apparatus or discharging the reaction fluid froma reactor in the second reaction apparatus, may preferably be {circlearound (1)} a method of feeding the reaction fluid from an upper portionof the reactor in the second reaction apparatus and discharging thereaction fluid continuously from a lower portion of the reactor, {circlearound (2)} a method of feeding the reaction fluid to a lower portion ofthe reactor by means of a supply tube from an upper portion of thereactor in the second reaction apparatus and continuously dischargingthe reaction fluid from an upper portion of the reactor, or {circlearound (3)} a method of feeding the reaction fluid from a lower portionof the reactor in the second reaction apparatus and continuouslydischarging the reaction fluid from an upper portion of the reactor.

The reaction fluid (slurry) discharged via the reactors of the secondreaction apparatus exhibits acidity due to the acid catalyst. In orderto neutralize this reaction fluid, an alkali neutralizing agent such assodium hydroxide or sodium bicarbonate, will be added. Usually, the pHis adjusted to be from 7 to 11.

The method of neutralizing the slurry may, for example, be a continuousneutralizing method wherein a suitable amount of alkali is continuouslyadded to the slurry discharged from the second reaction apparatus, or abatch system neutralizing method wherein the slurry discharged is oncestored in a tank, and then, a suitable amount of alkali is added withstirring.

Next, the treatment for removing a metal component by dehydration andwashing with water of the slurry thus obtained, and the drying method,are the same as those disclosed in the above (a).

(d) In this embodiment, water or an aqueous solution having an acidcatalyst dissolved therein, is preliminarily charged into a reactor,then polyvinyl alcohol, an aldehyde and an acid catalyst are fed, thenfeeding of the raw materials is stopped, and an acetalization reactionis carried out in the reactor, so that the acetalization degree ofpolyvinylacetal is adjusted to be at least 60 mol %.

Namely, there are characteristics in the apparatus and method forreacting polyvinyl alcohol, an aldehyde and an acid catalyst as rawmaterials for the polyvinylacetal resin. Firstly, it is preferred tosatisfy the following formula:V/v≧0.5where V is the charged amount (liters) of the water or the aqueoussolution having an acid catalyst dissolved therein, and v is the totalvolume per unit time (liters/hr) of the three types of the raw materialsfed.

By satisfying the above formula, sticking on the wall, of thepolyvinylacetal resin formed at the initial stage of the reaction in thereactor, can be suppressed. Then, in a stirred state, the polyvinylalcohol, the aldehyde and the acid catalyst are continuously fed to fillthe reactor, or before that, feeding of the raw materials is stoppedwhen the desired liquid level is reached, and then the acetalizationreaction and aging reaction are carried out in the reactor, so that theacetalization degree will reach at least 60 mol %.

The method of feeding the aqueous polyvinyl alcohol solution, thealdehyde and the acid catalyst into the reactor is not particularlylimited, and it is possible to employ {circle around (1)} a method ofseparately feeding the aqueous polyvinyl alcohol solution, the aldehydeand the acid catalyst from three inlets, {circle around (2)} a method ofpreliminarily mixing the polyvinyl alcohol and the acid catalyst, andthen feeding the aldehyde and the mixed fluid of the polyvinyl alcoholand the acid catalyst separately from the respective inlets, and {circlearound (3)} a method of preliminarily mixing the polyvinyl alcohol andthe aldehyde, and then feeding the acid catalyst and the mixed fluid ofthe polyvinyl alcohol and the aldehyde, separately from the respectiveinlets. Among them, from the viewpoint of the control of the reaction,the method {circle around (1)} or {circle around (2)} is preferred.

The reactor to be used in the present invention is preferably a tanktype reactor provided with a stirring mechanism, or a tubular reactor,with a view to reacting the aqueous polyvinyl alcohol solution, thealdehyde and the acid catalyst, thus fed, in a homogeneous system.Further, as the reactor, a reactor such as a full liquid type reactor oran open type reactor having a gas-liquid interface, may be used. As thestirring condition, it is preferred to carry out the stirring with astirring power per unit volume of at least 0.05 kW/m³ with a view toaccomplishing a proper mixing.

As stirring vanes to be used for stirring the reaction fluid in thereactor, those exemplified in the above (a) may be used. As the stirringsystem, with a view to suppressing sticking to the stirring vanes andthe stirring shaft, a lower stirring system is preferred. Further, alsowith respect to a baffle, a lower baffle system is preferred with a viewto suppressing the sticking.

In the present invention, the polyvinyl alcohol, the aldehyde and theacid catalyst are fed continuously or intermittently to fill thereactor, or the feeding is stopped before that when the desired liquidlevel has been reached, and the acetalization reaction is carried outwith stirring.

On the other hand, the reaction temperature for the acetalizationreaction is preferably from 20 to 50° C., as mentioned in the above (a),whereby a resin having good washability, can be obtained. The reactiontime is selected within a range of from 10 minutes to 10 hours,preferably from 30 minutes to 4 hours.

Then, the reaction fluid (slurry) discharged from the reactor istransferred to another reactor to carry out an aging reaction. The agingreaction temperature, time and desired acetalization degree are the sameas disclosed in the above (a). The treatment for neutralization of theslurry thus obtained, the treatment for removing metal by dehydrationand washing with water, and the drying method are also the same as thosedisclosed in the above (a).

Now, the present invention will be described in further detail withreference to Examples, but it should be understood that the presentinvention is by no means restricted to the following Examples.

EXAMPLE a1

An aqueous polyvinyl alcohol solution as one of the raw materials, wasprepared in the following manner. Into a 30 liter SUS dissolution tank,9000 parts of pure water and 1000 parts of polyvinyl alcohol having anaverage polymerization degree of 1700 and a saponification degree of 98mol %, were put and heated to completely dissolve the polyvinyl alcohol.

A cylindrical closed reactor made of glass and having a capacity of 2liters, which has three inlets at a lower portion of the reactor and hasone discharge outlet at an upper portion of the reactor, was ready. Intothe reactor, pure water was filled, and the internal temperature wasmaintained at 30° C. with stirring (anchor vanes, 400 rpm). During thereaction, the stirring state was maintained. Then, the above-mentioned10% polyvinyl alcohol aqueous solution, 35% hydrochloric acid as an acidcatalyst, and butyl aldehyde (purity: 99.5%) as an aldehyde, were madeready and then fed from a lower portion of the reactor, so that therespective supply rates would be 60 kg/hr, 1.9 kg/hr and 4.5 kg/hr,respectively, to carry out the acetalization reaction. After theacetalization degree of the formed polyvinylacetal reached at least 10mol %, the reaction fluid was discharged from an upper portion of thereactor, while feeding the above-mentioned aqueous polyvinyl alcoholsolution, hydrochloric acid and butyl aldehyde from a lower portion ofthe reactor.

The discharged reaction product (slurry) was transferred to a 10 literaging tank separately made ready (the transported amount: 5 kg) and thenaged at 50° C. for two hours. The stirring vanes of the aging tank werethree-bladed sweptback vanes, and a condition of rotational speed forstirring of 250 rpm was employed. Further, the average retention time ofthe reactor at that time was two minutes, and the discharged fluid fromthe closed reactor was sampled and measured, whereby the butyralizationdegree at the outlet of the reactor was 41 mol %, and the butyralizationdegree after aging for two hours at 50° C., was 68 mol %.

Then, an aqueous sodium hydroxide solution was added to the slurry toadjust the pH to 8. After cooling to room temperature, this slurry wasdehydrated by a centrifugal separator to a water content of 45%, andwater in an amount of ten times to the resin content was added fordilution and stirred for 30 minutes for washing with water.

This operation of dehydration and washing with water, was repeated threetimes, and the obtained slurry was again dehydrated and then dried toobtain a polyvinylbutyral resin as a white powder. Further, thetemperature of water employed for the washing with water was 25° C. ineach case.

The butyralization degree of the obtained polyvinylbutyral resin was 68mol %. The specific surface area per unit weight of the resin powdermeasured by means of an automatic porosimeter autopore IV9500,manufactured by Shimadzu Corporation, was 2.6 m²/g. The content ofsodium element in the resin, as measured by ICP emission elementalanalysis, was 20 ppm.

The results of the butyralization degree of the product sampled at theoutlet of the reactor, the butyralization degree of the finally obtainedpolyvinylbutyral resin, the specific surface area of the resin powder asmeasured by the porosimeter, the bulk density, the particle diameter andthe amount of sodium in the resin measured by ICP, are summarized inTable 1.

EXAMPLE a2

The operation was carried out in the same manner as in Example a1 exceptthat in Example a1, the temperature of the reactor was changed to 20° C.The butyralization degree of the reaction product sampled at thedischarge outlet of the reactor was 19 mol %.

EXAMPLE a3

The operation was carried out in the same manner as in Example a1 exceptthat in Example a1, the feeding rate of the three types of raw materialsinto the reactor was changed to twice (i.e. an average retention time ofone minute). The butyralization degree of the reaction product sampledat the discharge outlet of the reactor, was 25 mol %.

EXAMPLE a4

The operation was carried out in the same manner as in Example a1 exceptthat in Example a1, the method of feeding the three types of rawmaterials into the reactor was changed as follows.

Firstly, the 10% polyvinyl alcohol aqueous solution and 35% hydrochloricacid were preliminarily mixed in a predetermined ratio. This mixed fluidand butyl aldehyde were fed from two inlets at a lower portion of thereactor at rates of 61.9 kg/hr and 4.5 kg/hr, respectively, and thereaction fluid wherein the acetalization degree of polyvinylacetalformed, reached at least 10 mol %, was discharged from an upper portionof the reactor, while feeding the above polyvinyl alcohol aqueoussolution, hydrochloric acid and butyl aldehyde from a lower portion ofthe reactor. The butyralization degree of the reaction product sampledat the discharge outlet of the reactor, was 39 mol %.

EXAMPLE a5

The operation was carried out in the same manner as in Example a4 exceptthat in Example a4, the mixed fluid comprising the polyvinyl alcoholaqueous solution and 35% hydrochloric acid, was fed to a lower portionof the reactor via a nozzle inserted from an upper portion of thereactor (the forward end of the nozzle was set at a position of H/5 fromthe bottom where H is the height of the reactor), and the reaction fluidwas discharged from a discharge outlet at an upper portion of thereactor.

The butyralization degree of the reaction product sampled at thedischarge outlet of the reactor, was 35 mol %.

EXAMPLE a6

The operation was carried out in the same manner as in Example a4 exceptthat in Example a4, the mixed fluid comprising the polyvinyl alcoholaqueous solution and 35% hydrochloric acid, and the butylaldehyde werefed from two inlets at an upper portion of the reactor, and the reactionproduct was discharged from the bottom of the reactor. Thebutyralization degree of the reaction product sampled at the dischargeoutlet of the reactor, was 37 mol %.

EXAMPLE a7

The operation was carried out in the same manner as in Example a4 exceptthat in Example a4, max blend vanes (manufactured by Sumitomo HeavyIndustries, Ltd., ratio of the vane width to the inner diameter of thereactor: 0.55) were used as the stirring vanes for the reactor.

The butyralization degree of the reaction product sampled at thedischarge outlet of the reactor, was 43 mol %.

EXAMPLE a8

The operation was carried out in the same manner as in Example a4 exceptthat in Example a4, two paddle vanes (ratio of the paddle width to theinner diameter of the reactor: 0.6) were used as the stirring vanes forthe reactor, and the condition for the stirring speed was changed to 120rpm.

The butyralization degree of the reaction product sampled at thedischarge outlet of the reactor, was 35 mol %.

COMPARATIVE EXAMPLE a1

A glass reactor having an internal capacity of 2 liters which wassimilar to Example a1 but which had no inlet at a lower portion of thereactor, was made ready. A 10% polyvinyl alcohol aqueous solution, 35%hydrochloric acid and butylaldehyde were made ready under such acondition that the feeding amounts per unit time into the reactor wouldbe the same ratio as in Example a1, i.e. 600 g, 19 g and 45 g,respectively. In a state where the stirring vanes of the reactor wererotated, the above three types of raw materials were addedsimultaneously from separate inlets at an upper portion of the reactor.Upon expiration of two minutes, the mixture was quickly transferred to aseparate aging tank and then aged.

The butyralization degree of the reaction fluid sampled immediatelyafter expiration of two minutes, was 49 mol %. Thereafter, the procedureof Example a1 was followed to finally obtain a polyvinylbutyral resinpowder.

COMPARATIVE EXAMPLE a2

In Comparative Example a1, a mixed fluid comprising a 10% polyvinylalcohol aqueous solution and 35% hydrochloric acid, was preliminarilyprepared, and 619 g of this mixed fluid and 45 g of butylaldehyde wereadded.

EXAMPLE b1

An aqueous polyvinyl alcohol solution as one of the raw materials, wasprepared as follows.

Into a 150 liter SUS dissolution tank, 90000 parts of pure water and10000 parts of polyvinyl alcohol having an average polymerization degreeof 1700 and a saponification degree of 98.5 mol %, were put and heatedto completely dissolve the polyvinyl alcohol.

A cylindrical closed reactor made of glass and having a capacity of 9liters (provided with two rod baffles) which had three inlets at a lowerportion of the reactor and one discharge outlet at an upper portion ofthe reactor, was made ready. Pure water was filled into the reactor andthe internal temperature was maintained at 32° C. with stirring (anchorvanes, 350 rpm).

The above-mentioned 10% polyvinyl alcohol aqueous solution, 35%hydrochloric acid as an acid catalyst and butylaldehyde (purity: 99.5%)as an aldehyde, were made ready. While feeding them from a lower portionof the reactor so that the respective feeding rates would be 9.0 kg/hr,0.29 kg/hr and 0.68 kg/hr, the formed polyvinylacetal reaction fluid(slurry) was discharged from an upper portion of the reactor.

Feeding of the raw materials into the reactor was carried out for 5hours, and then the feeding line of the polyvinyl alcohol aqueoussolution was switched to pure water. The interior of the reactor wassufficiently substituted by pure water, and then, the reactor wasdisassembled, whereupon the sticking state on the inner surface of thereactor, the stirring vanes and the baffle was ascertained. As a result,no substantial sticking was observed on the inner surface of thereactor, on the stirring vanes or on the baffle, and the condition wasgood. The total weight of deposits sticking on the inner wall, thevanes, the shaft and the baffle, was 58 g.

The average retention time in the reactor under this condition was 60minutes, and the acetalization degree of the formed polyvinylacetalsampled at the discharge outlet at the upper portion of the reactor, was58 mol %. A part of the obtained slurry was transferred to a separate 6liter aging tank (amount transferred: 3 kg) and then aged at 55° C. fortwo hours. The stirring vanes of the aging tank were three-bladedsweptback vanes, and a condition of the stirring rotational speed of 150rpm was adopted.

Then, an aqueous sodium hydroxide solution was added to adjust the pH to9. After cooling to room temperature, this slurry was dehydrated by acentrifugal separator to a water content of 45%, then diluted by anaddition of water in an amount of ten times to the resin content andwashed with water with stirring for 30 minutes.

This operation of dehydration and washing with water was repeated threetimes, and the obtained slurry was again dehydrated and then dried toobtain a white powdery polyvinylbutyral resin. Here, the temperature ofwater used for washing was 25° C. each time.

The butyralization degree of the obtained polyvinylbutyral resin was 70mol %. The specific surface area per unit weight of the resin powdermeasured by means of automatic porosimeter autopore IV500, manufacturedby Shimadzu Corporation, was 3.2 m²/g.

The content of sodium element in the resin was 18 ppm as measured as anICP emission elemental analysis.

The results of the butyralization degree of the reaction product sampledat the discharge outlet of the reactor, the butyralization degree of thefinally obtained polyvinylbutyral resin, the thickness of the depositsticking to the inner surface of the reactor, the specific surface areaof the resin powder measured by a porosimeter, the particle size and theamount of sodium in the resin measured by ICP, are summarized in Table2.

EXAMPLE b2

The operation was carried out under the same conditions as in Exampleb1, except that the temperature in the reactor was changed to 25° C.

EXAMPLE b3

In Example b1, the operation was carried out by adjusting the feedingrates of the polyvinyl alcohol aqueous solution, 35% hydrochloric acidand butylaldehyde to be 4.5 kg/hr, 0.145 kg/hr and 0.34 kg/hr,respectively (the average retention time: 120 minutes).

EXAMPLE b4

In Example b1, the operation was carried out by adjusting the feedingrates of the polyvinyl alcohol aqueous solution, 35% hydrochloric acidand butylaldehyde to be 2.25 kg/hr, 0.073 kg/hr and 0.17 kg/hr,respectively (the average retention time: 240 minutes), and at areaction temperature of 37° C.

EXAMPLE b5

A tank type reactor made of glass and having a capacity of 9 liters, wasmade ready, and a glass nozzle having an inner diameter of 20 mm wasattached at a position where the internal capacity would be 4 liters(side of the reactor). Three supply tubes were attached at threepositions at an upper portion of the reactor, and via such supply tubes,three types of raw materials i.e. a 10% polyvinyl alcohol aqueoussolution, 35% hydrochloric acid and butylaldehyde, were, respectively,fed from a lower portion of the reactor. The respective feeding rateswere 6.0 kg/hr, 0.19 kg/hr and 0.45 kg/hr (the average retention time:40 minutes). During the feeding, the internal temperature was maintainedto be 45° C. with stirring (anchor vanes, 100 rpm). The formedpolyvinylacetal reaction fluid was continuously discharged from thenozzle at the side of the reactor.

EXAMPLE b6

In Example b5, the operation was carried out by attaching the glassnozzle at a position where the internal capacity would be 6 liters(average retention time: 60 minutes).

EXAMPLE b7

In Example b5, the operation was carried out by attaching the glassnozzle at a position where the internal capacity would be 6 liters(average retention time: 60 minutes), and adjusting the feeding rates ofthe 10% polyvinyl alcohol aqueous solution, 35% hydrochloric acid andbutylaldehyde to be 3.0 kg/hr, 0.095 kg/hr and 0.225 kg/hr, respectively(the average retention time: 120 minutes), and at a reaction temperatureof 20° C.

EXAMPLE b8

In Example b7, the operation was carried out at a reaction temperatureof 50° C.

COMPARATIVE EXAMPLE b1

A glass reactor having an internal capacity of 2 liters, which wassimilar to Example b1 but which has no inlet at a lower portion of thereactor, was made ready. A 10% polyvinyl alcohol aqueous solution, 35%hydrochloric acid and butylaldehyde were made ready under suchconditions that the feeding volumes into the reactor per unit time wouldbe in the same ratio as in Example b1, i.e. 900 g, 29 g and 68 g,respectively. In a state where the stirring vanes of the reactor wererotated, the above three types of raw materials were simultaneouslyadded from separate inlets at an upper portion of the reactor. Uponexpiration of 60 minutes, the mixture was transferred to a separateaging tank and then aged. The butyralization degree of the reactionfluid sampled immediately after expiration of 60 minutes, was 53 mol %.

Thereafter, the procedure of Example b1 was followed to finally obtain apolyvinylbutyral resin powder.

COMPARATIVE EXAMPLE b2

The operation was carried out under the same conditions as in Exampleb1, except that a cylindrical closed reactor made of glass and having acapacity of 2 liters (provided with two rod baffles) was employed. Theaverage retention time in the reactor was about 13 minutes. Afterdisassembling the reactor, the sticking state was observed, wherebysticking was substantial particularly on the baffle and anchor vanes.The total weight of the deposits was 210 g.

COMPARATIVE EXAMPLE b3

The operation was carried out under the same conditions as in Exampleb1, except that a cylindrical closed reactor made of glass and having acapacity of 4 liters (provided with two rod baffles) was employed. Theaverage retention time in the reactor was about 26 minutes. Afterdisassembling the reactor, the sticking state was observed, wherebysticking was observed on the baffle, anchor vanes and inner wall of thereactor. The total weight of the deposits was 107 g.

COMPARATIVE EXAMPLE b4

In Example b6, the operation was carried out at a reaction temperatureof 5° C.

COMPARATIVE EXAMPLE b5

In Example b6, the operation was carried out at a reaction temperatureof 65° C. Upon expiration of 14 minutes after initiation of feeding theraw materials, the discharge nozzle at the side of the reactor wasclogged, and the reaction fluid was not constantly discharged.Therefore, the operation was stopped.

EXAMPLE c1

An aqueous polyvinyl alcohol solution was prepared as follows. Into a600 liter SUS dissolution tank, 400000 parts of pure water and 40000parts of polyvinyl alcohol having an average polymerization degree of1700 and a saponification degree of 98.5 mol %, were put and heated tocompletely dissolve the polyvinyl alcohol.

As the reactor of the first reaction apparatus, a tank type closedreactor made of glass and having a capacity of 500 ml which had threeinlets at a lower portion of the reactor and one discharge outlet at anupper portion of the reactor, was used. Pure water was filled into thereactor of the first reaction apparatus, and the internal temperaturewas maintained to be 30° C. with stirring (anchor vanes, 400 rpm).

Then, the above-mentioned polyvinyl alcohol aqueous solution, 35%hydrochloric acid as an acid catalyst, and butyl aldehyde (purity:99.5%) as an aldehyde, were used and then fed from a lower portion ofthe reactor, so that the respective feeding rates would be 15 kg/hr, 0.5kg/hr and 1.1 kg/hr, respectively, to carry out the acetalizationreaction, and the first reaction fluid was continuously discharged froman upper portion of the reactor of the first reaction apparatus (theaverage retention time of the first reaction fluid in the reactor of thefirst reaction apparatus: about 1.8 minutes).

Then, the first reaction fluid was fed to a lower portion of the firstreactor (outer temperature: 40° C., stirring with anchor vanes: 400 rpm)among three closed reactors made of glass and having an internalcapacity of 15 liters (the reactors of the second reaction apparatus)disposed in series with the reactor of the first reaction apparatus,while the reaction fluid was discharged from an upper portion of thereactor and then carrying out feeding and discharging in a similarmanner to a second reactor and a third reactor (the total of the averageretention times of the three reactors of the second reaction apparatus:2.7 hours) to continuously carry out an aging reaction in the reactorsof the second reaction apparatus to finally have the second reactionfluid discharged from an upper portion of the third reactor of thesecond reaction apparatus.

The second reaction fluid (slurry) discharged from the reactor of thesecond reaction apparatus was stored in a tank of 500 liters. Then, anaqueous sodium hydroxide solution was added to the slurry to adjust thepH to 8.

After cooling to room temperature, this slurry was dehydrated by acentrifugal separator to a water content of 48%, and water was added inan amount of 15 times to the resin component, whereupon washing withwater was carried out by stirring for 30 minutes.

This operation of dehydration and washing with water was repeated twice,and the obtained slurry was dehydrated again and then dried to obtain awhite powdery polyvinylbutyral resin. Here, the temperature of water tobe used for washing with water was 25° C. in each case.

The content of sodium element in the resin as measured by an ICPemission elemental analysis, was 13 ppm. Here, the specific procedure ofthe ICP analysis was as follows. 0.5 g of the polyvinylbutyral resin and5 ml of nitric acid were put into a microwave decomposition container(inner cylinder) and decomposed by a microwave decomposition apparatus(MLS-1200MEGA, manufactured by Milestone). After cooling, the entiredecomposition container was dried up in a water bath, and 20 ml of 3.5%hydrochloric acid was added, followed by heating for dissolution. Sodiumin this solution was quantified by the ICP emission spectroscopicanalyzer (SPS-1200A, manufactured by Seiko Instruments Inc., measuringcondition: plasma power 0.9 kW, wavelength 589 nm).

The results of the butyralization degree of the obtainedpolyvinylbutyral resin and the amount of sodium in the resin measured bythe ICP analysis, are summarized in Table 3.

EXAMPLE c2

In Example c1, the feeding rates of the polyvinyl alcohol aqueoussolution, 35% hydrochloric acid and butylaldehyde were adjusted to be 10kg/hr, 0.33 kg/hr and 0.73 kg/hr, respectively (total of the respectiveaverage retention times in the reactors of the second reactionapparatus: about 4.1 hours).

EXAMPLE c3

In Example c1, the feeding rates of the polyvinyl alcohol aqueoussolution, 35% hydrochloric acid and butylaldehyde were adjusted to be 20kg/hr, 0.67 kg/hr and 1.8 kg/hr, respectively (total of the respectiveaverage retention times in the reactors of the second reactionapparatus: about 2.0 hours).

EXAMPLE c4

In Example c1, the feeding rates of the polyvinyl alcohol aqueoussolution, 35% hydrochloric acid and butylaldehyde were adjusted to be 10kg/hr, 0.33 kg/hr and 0.73 kg/hr, respectively, and two reactors of thesecond reaction apparatus, having an internal capacity of 15 liters wereused in series (total of the respective average retention times in thereactors of the second reaction apparatus: about 2.7 hours), and thereaction fluid was continuously discharged.

EXAMPLE c5

In Example c1, one reactor having an internal capacity of 50 liters, ofthe second reaction apparatus, was used, and the reaction fluid was fedfrom a lower portion of this reactor, and the slurry was continuouslydischarged from an upper portion of the reactor (the average retentiontime in the reactor of the second reaction apparatus: about 3.0 hours).

EXAMPLE c6

An aqueous polyvinyl alcohol solution was prepared as follows. Into aSUS dissolution tank of 2000 liters, 1656000 parts of pure water and144000 parts of polyvinyl alcohol having an average polymerizationdegree of 1700 and a saponification degree of 98.8 mol %, were put andheated to completely dissolve the polyvinyl alcohol.

As the reactor of the first reaction apparatus, a tank type closedreactor made of glass and having a capacity of 2 liters, which had threeinlets at a lower portion of the reactor and one discharge outlet at anupper portion of the reactor (the reactor of the first reactionapparatus), was made ready. Pure water was filled into the reactor ofthe first reaction apparatus, and the internal temperature wasmaintained to be 30° C. with stirring (anchor vanes, 400 rpm).

Then, the above-mentioned polyvinyl alcohol aqueous solution, 35%hydrochloric acid as an acid catalyst, and butyl aldehyde (purity:99.5%) as an aldehyde, were made ready, and they were fed from a lowerportion of the reactor, so that the respective feeding rates would be 30kg/hr, 1.0 kg/hr and 2.1 kg/hr, to carry out the acetalization reaction,and the first reaction fluid was continuously discharged from an upperportion of the reactor (the average retention time in the reactor of thefirst reaction apparatus: about 3.6 minutes).

Then, the first reaction fluid was fed in the same procedure as inExample c1 into eight closed reactors made of glass and having aninternal capacity of 10 liters disposed in series as reactors of thesecond reaction apparatus (outer temperature: 35° C., stirring withanchor vanes: 300 rpm), while the reaction fluid was discharged from anupper portion of the reactors, so that while continuously carrying outthe aging reaction in the second reaction apparatus, the second reactionfluid was finally discharged from an upper portion of the 8th reactor ofthe second reaction apparatus (total of the respective average retentiontimes in the reactors of the second reaction apparatus: about 2.4hours).

The second reaction fluid (slurry) discharged from the second reactionapparatus was stored in a tank of 2000 liters. Then, an aqueous sodiumhydroxide solution was added to the slurry to adjust the pH to 8.Thereafter, in the same procedure as in Example c1, a polyvinylbutyralresin was obtained.

COMPARATIVE EXAMPLE c1

A glass reactor having an internal capacity of 15 liters, which wassimilar to Example c1 but which has no inlet at a lower portion of thereactor, was made ready. A 9% polyvinyl alcohol aqueous solution, 35%hydrochloric acid and butylaldehyde were made ready under such acondition that the feeding amounts into the reactor per unit hour wouldbe the same ratio as in Example c1, i.e. 6000 g, 200 g and 440 g,respectively. In a state where the stirring vanes of the reactor wererotated, the three types of raw materials were simultaneously addedbatchwise from separate inlets at an upper portion of the reactor. Uponexpiration of 1.8 minutes, the mixture was quickly transferred to aseparate aging tank made of glass and having an internal capacity of 15liters batchwise. An aging reaction was carried out by maintaining at40° C. for 2.7 hours with stirring (anchor vanes: 400 rpm). Thereafter,in accordance with the procedure as in Example c1, a polyvinylbutyralresin was obtained.

EXAMPLE d1

An aqueous polyvinyl alcohol solution as one of the raw materials wasprepared as follows. Into a SUS dissolution tank of 15 liters, 9000parts of pure water and 1000 parts of polyvinyl alcohol having anaverage polymerization degree of 1800 and a saponification degree of99.1 mol %, were put and heated to completely dissolve the polyvinylalcohol. Then, the aqueous polyvinyl alcohol solution was maintained at50° C.

A tank type reactor made of glass and having a capacity of 6 liters,provided with a stirring mechanism (stirring vanes: anchor vanes made ofTeflon (registered trademark) with d/D=0.65), was made ready. Threesupply tubes serving also as baffles, were attached from three positionsat an upper portion of the reactor. One liter of pure water was charged,and the temperature was adjusted at 35° C. Then, with stirring (stirringrotational speed: 65 rpm), the 10% polyvinyl alcohol aqueous solution,butylaldehyde and 20% hydrochloric acid were fed from the respectivesupply tubes. The respective feeding rates were 16.7 ml/min, 1.30 ml/minand 1.09 ml/min (the feeding volumes of the raw materials per unit hour:1.15 liters/hr, V/v≈0.0.87). During the feeding, the stirring rotationalspeed was gradually raised (from 65 to 120 rpm) with an increase of theliquid surface.

During this series of operations, the internal temperature wascontrolled by using a jacket of the reactor and maintained at 30° C.When the total amount fed into the reactor became 4 liters (the totalliquid amount including the preliminarily charged 20% hydrochloric acid:5 liters), feeding of the raw materials was stopped. In this state, theacetalification reaction was continued for one hour. Upon expiration ofone hour, warm water was circulated into the jacket to heat the reactor,and the internal temperature was maintained at 55° C. In this state, thestirring rotational speed was raised from 120 to 140 rpm and maintainedfor two hours. Thus, a polyvinylacetal (slurry) was obtained.

Then, an aqueous sodium hydroxide solution was added to adjust the pH to9. After cooling to room temperature, this slurry was dehydrated by acentrifugal separator to a water content of 45% and then diluted by anaddition of water in an amount of 10 times to the resin component, andwashed with water by stirring for 30 minutes. This operation ofdehydration and washing with water was repeated three times, and theobtained slurry was dehydrated again and then dried to obtain a whitepowdery polyvinylbutyral resin. Here, the temperature of water used forwashing with water, was 25° C. in each case.

The specific surface area per unit weight of the resin powder measuredby means of automatic porosimeter autopore IV500, manufactured byShimadzu Corporation, was 3.3 m²/g.

The content of sodium element in the resin as measured by an ICPemission elemental analysis, was 11 ppm.

The results of the butyralization degree of the obtainedpolyvinylbutyral resin, the specific surface area of the resin powdermeasured by the porosimeter, the particle size and the amount of sodiumin the resin measured by ICP, are summarized in Table 4.

EXAMPLE d2

The operation was carried out under the same conditions as in Example d1except that the internal temperature of the reactor was changed to 25°C., and 1 liter of 1% hydrochloric acid was preliminarily charged intothe reactor.

EXAMPLE d3

In Example d1, the operation was carried out by adjusting the feedingrates of the polyvinyl alcohol aqueous solution, butylaldehyde and 20%hydrochloric acid to be 9.8 ml/min, 0.77 ml/min and 0.64 ml/min (thevolume of the raw materials fed per unit hour: 0.67 liters/hr,V/v≈1.49).

EXAMPLE d4

In Example d1, the operation was carried out by using three-bladedsweptback vanes (d/D=0.64) and medium paddle vanes (d/D=0.60) asstirring vanes.

EXAMPLE d5

In Example d1, the operation was carried out by using a tank typereactor made of glass and having a capacity of 6 liters, which wasprovided with a stirring mechanism at a lower portion of the reactor(stirring vanes: anchor vanes made of Teflon (registered trademark) withd/D=0.65).

EXAMPLE d6

An aqueous polyvinyl alcohol solution as one of the raw materials wasprepared as follows. Into a SUS dissolution tank of 2 m³, 900000 partsof pure water and 100000 parts of polyvinyl alcohol having an averagepolymerization degree of 1800 and a saponification degree of 99.0 mol %,were put and heated to completely dissolve the polyvinyl alcohol. Then,the mixture was maintained at 45° C.

A tank type reactor having a capacity of 1 m³ (glass lining on the innersurface) which was provided with a stirring mechanism (stirring vanes:max blend vanes (d/D=0.55), manufactured by Sumitomo Heavy Industries,Ltd., and coated with Teflon (registered trademark)) was made ready.From three positions at an upper portion of the reactor, three supplytubes serving also as baffles, were attached. 150 liters of pure waterwas charged, and the temperature was adjusted at 32° C.

Then, with stirring (stirring rotational speed: 50 rpm, power per unitvolume Pv=0.14 kW/m³), a 10% polyvinyl alcohol aqueous solution,butylaldehyde and 35% hydrochloric acid were fed from the respectivesupply tubes. The respective feeding rates were 100 liters/hr, 7.7liters/hr and 3.1 liters/hr, respectively (the total volume fed per unithour: 110.8 liters/hr, V/v≈1.35). During the feeding, the stirringrotational speed was gradually raised (from 50 to 60 rpm, power per unitvolume at termination of feeding of the raw materials: Pv=0.14 kW/m³),as the liquid level increased. During this period, the internaltemperature was controlled at 32° C. by cooling with water through thejacket of the reactor. When the total amount of the liquid in thereactor reached 900 liters, feeding of the raw materials was stopped. Inthis state, the acetalization reaction was continued for one hour.

Then, the internal temperature of the reactor was raised to 55° C. andmaintained for two hours to carry out the aging reaction. Thus, apolyvinylacetal (slurry) was obtained. Thereafter, in the same procedureas in Example d1, a polyvinylacetal powder was prepared.

COMPARATIVE EXAMPLE d1

A glass reactor having an internal capacity of 2 liters, which wassimilar to Example d1 but which had no inlet at a lower portion of thereactor, was made ready. A 10% polyvinyl alcohol aqueous solution,butylaldehyde and 20% hydrochloric acid were made ready under such acondition that the amounts fed into the reactor per unit time would bethe substantially the same ratio as in Example d1, i.e. 900 g, 68 g and59 g, respectively. In a state where the stirring vanes of the reactorwere rotated, the three types of raw materials were added simultaneouslyfrom separate inlets at an upper portion of the reactor. Upon expirationof one hour, the inner temperature was raised to 55° C. and maintainedfor two hours to carry out an aging reaction. Thereafter, in accordancewith the procedure in Example d1, a polyvinylbutyral resin powder wasfinally obtained.

COMPARATIVE EXAMPLE d2

In Example d1, the operation was carried out without preliminarilycharging pure water into the reactor. Immediately after feeding thepolyvinyl alcohol aqueous solution, butylaldehyde and 20% hydrochloricacid, white blocks of a resin were formed at the bottom of the reactor,and the inlets of the raw materials were clogged.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a porouspolyvinylacetal resin powder having a specific surface area of from 1.5to 3.5 m²/g, which has a very low content of a metal component such asan alkali metal and thus is excellent in transparency, moistureresistance and electrical insulating properties and whereby sticking tothe production equipment such as the reactor or piping is scarce.

Further, the present invention provides a process whereby theabove-mentioned polyvinylbutyral resin of high quality can becontinuously produced constantly over a long period of time by aproduction equipment smaller than before or by an economical productionequipment having freeness in design or installation site increased,while suppressing deposition of the polyvinylacetal resin on thereactor, the piping or the like.

TABLE 1 Ex. Comp. Ex. a1 a2 a3 a4 a5 a6 a7 a8 a1 a2 Butyralization 41 1925 39 35 37 43 35 49 47 degree of the reaction product at the dischargeoutlet of the reactor (mol %) Butyralization 68 65 66 68 67 67 68 66 6969 degree of the resin powder (mol %) Specific 2.6 2.5 2.7 3.0 2.8 2.83.0 1.7 0.7 0.9 surface area (m²/g) Bulk density 0.16 0.16 0.15 0.150.16 0.16 0.14 0.19 0.21 0.20 (g/cm³) Particle size 1.3 1.3 1.3 1.2 1.31.4 1.2 2.4 4.8 5.3 (μm) Amount of 20 45 57 31 29 15 11 78 118 122sodium in the resin (ppm)

TABLE 2 Ex. Comp. Ex. b1 b2 b3 b4 b5 b6 b7 b8 b1 b2 b3 b4 b5 Averageretention 60 60 120 240 40 60 120 120 — 13 26 60 60 time in the reactor(min) Internal temperature 32 25 32 37 45 45 20 50 32 32 32 5 65 of thereactor (° C.) to 37* Butyralization 58 52 60 61 61 61 54 63 53 45 47 28—** degree of the reaction fluid at the discharge outlet of the reactor(mol %) Butyralization 70 69 71 71 71 72 68 72 67 68 68 66 — degree ofthe resin powder (mol %) Specific surface 3.2 3.3 3.1 3.3 2.8 2.8 3.32.3 0.8 2.8 2.9 1.2 — area (m²/g) Particle diameter 1.1 1.2 1.2 1.3 1.21.2 0.9 1.9 5.1 1.3 1.2 3.0 — (μm) Amount of sodium in 18 15 15 13 18 1120 29 137 17 20 120 — the resin (ppm) Weight of deposit 58 39 37 19 6560 67 72 —*** 210 107 141 — upon expiration of 5 hours of feeding (g)*The temperature increased to 37° C. by heat generation during thereaction. **The operation was stopped, as the overflow piping wasclogged. ***Not measured because of the batch reaction.

TABLE 3 Comp. Ex. Ex. c1 c2 c3 c4 c5 c6 c1 Volume of the 0.5 2 15reactor of the first reaction apparatus (liters) Average retention 1.82.7 1.3 2.7 1.8 3.6 1.8 time (min) Number of 3 3 3 2 1 8 1 reactors ofthe second reaction apparatus Each volume 15 15 15 15 50 10 15 (liters)Total of average 2.7 4.1 2.0 2.7 3.0 2.4 2.7 retention times (hr)Butyralization 68 69 65 69 68 68 68 degree at the discharge outlet ofthe reactor of the second reaction apparatus (mol %) Amount of sodium 1311 17 12 17 9 123 in the resin (ppm)

TABLE 4 Ex. Comp. Ex. d1 d2 d3 d4 d5 d6 d1 d2 Butyralization 72 71 72 7272 72 68 —* degree of the resin powder (mol %) Specific 3.3 3.2 3.1 3.33.1 3.4 0.7 —  surface area (m²/g) Particle size 1.0 1.0 1.0 1.1 1.0 0.95.0 —  (μm) Amount of 9 7 11 10 15 13 109 —  sodium in the resin (ppm)*No powder was obtained.

1. A polyvinylacetal resin obtained by reacting polyvinyl alcohol and analdehyde in the presence of an acid catalyst, and wherein the resin hasan acetalization degree of at least 60 mol % and a specific surface areaof from 1.50 to 3.50 m²/g a bulk density of from 0.12 to 0.19 g/cm³, anaverage particle diameter of from 0.5 to 2.5 μm and a metal content ofat most 80 ppm.
 2. A process for producing a polyvinylacetal resin,comprising feeding a reaction fluid comprising polyvinyl alcohol, analdehyde and an acid catalyst into a first reactor, to carry out anacetalization reaction, discharging the reaction fluid wherein theacetalization degree has reached from 10 to 60 mol %, and feeding thesame into a second reactor to carry out a further reaction to bring theacetalization degree of polyvinylacetal to at least 65 mol %, whereinthe polyvinylacetal resin has a specific surface area of from 1.50 to3.50 m²/g a bulk density of from 0.12 to 0.19 g/cm³, an average particlediameter of from 0.5 to 2.5 μm and a metal content of at most 80 ppm. 3.The process for producing a polyvinylacetal resin according to claim 2,wherein the first reactor is a closed reactor provided with a stirringmechanism.
 4. The process for producing a polyvinylacetal resinaccording to claim 2, wherein the reaction temperature in the firstreactor is within a range of from 10 to 60° C.
 5. The process accordingto claim 4, wherein the reaction temperature in the first reactor isfrom 20 to 50° C.
 6. The process according to claim 4, wherein thereaction temperature in the first reactor is from 25 to 45° C.